Conventionally, when attaching an imaging lens to a camera main body which has a different effective screen size and a different mount, a mount conversion adaptor lens must be attached between the imaging lens and the mount. Such an adaptor lens is disclosed in Japanese Laid Open Patent Application 11-142732, which attaches an imaging lens to a camera which has a smaller effective screen size than the proper image size of the lens, and consists of four lens elements of negative, positive, negative, and positive refractive power, in sequential order from the object side. This lens provides a sufficient back focus for insertion of a glass prism between the last element of the lens and the image plane, while favorably correcting the spherical aberration and chromatic aberration which otherwise would be caused by the color separation prism. Because the adaptor lens provides its own magnification (which is unity or higher), there is a problem in that, if the adaptor lens is attached, the composite focal length of the optical system becomes longer and the image angle (i.e., the picture angle) becomes smaller than that of the imaging lens alone.
The object of the present invention is to provide a mount conversion adaptor lens which provides only a small change in the image angle and which favorably corrects the various aberrations even when an imaging lens used for TV broadcasting and so on, is attached to a camera designed for a different image size, using a C mount, or a CS mount.
The mount conversion adaptor lens of the present invention (hereinafter referred to simply as an adaptor lens) is attached between an imaging lens and a mount of a camera and is formed of a front lens group and a rear lens group, in order from the object side. The front lens group includes, in order from the object side, a negative lens element and at least one meniscus lens element with its concave surface on the object side. The rear lens group includes, in sequential order from the object side, a positive lens element with its convex surface on the object side, and a negative lens element.